The warning comes after the plant was rocked by a third blast which appears to have damaged one of the reactors' containment systems for the first time.If it is breached, there are fears of more serious radioactive leaks.Officials have extended the danger zone, warning residents within 30km (18 miles) to evacuate or stay indoors.

Analysis

Richard BlackEnvironment correspondent, BBC News

It appears that for the first time, the containment system around one of the Fukushima Daiichi reactors has been breached.Officials have referred to a possible crack in the suppression chamber of reactor 2 - a large doughnut-shaped structure below the reactor housing. That would allow steam, containing radioactive substances, to escape continuously.This is the most likely source of the high radioactivity readings seen near the site. Another possible source is the fire in reactor 4 building - believed to have started when a pool storing old fuel rods dried up.The readings at the site rose beyond safe limits - 400 millisieverts per hour (mSv/hr), when the average person's exposure is 3mSv in a year.A key question is whether this is just a transient spike, which might be expected if number 2 is the source, or whether the high levels are sustained.In the meantime, the key task for workers at the plant remains to get enough water into the reactors - and, now, into the spent fuel pools - with the poor resources at their disposal.

The crisis has been prompted by last Friday's 9.0-magnitude quake and tsunami in north-eastern Japan.

On Tuesday morning, reactor 2 became the third to explode in four days at the Fukushima Daiichi plant.A fire also briefly broke out at the plant's reactor 4, and is believed to have caused radioactive leaks. Reactor 4 had been shut down before the quake for maintenance, but its spent nuclear fuel rods are still stored on the site.Radiation levels in the Japanese capital - 250km (155 miles) away - were reported to be higher than normal, but officials said there were no health dangers.Tokyo residents have been stocking up on supplies, with some stores selling out of items such as food, water, face masks and candles.Housewife Mariko Kawase, 34, told AFP news agency: "I am shopping now because we may not be able to go out due to the radiation." In other developments:

A 70-year-old woman has been rescued alive from rubble in the coastal town of Otsuchi, five days after disaster

The Nikkei share index tumbled again, ending 10.55% lower, as the central bank pumped almost $100bn (£62bn) more cash into the financial system, a day after its record $183bn intervention

In a televised address, Prime Minister Naoto Kan said: "There is still a very high risk of more radiation coming out."No-fly zone He said that those living within between 20km (12 mile) and 30km of the plant were at risk and should not leave their homes.

“Start Quote

Do not go outside”

End QuoteYukio EdanoChief Cabinet Secretary

Residents within 20km have already been advised to evacuate, and the premier said anyone left in that exclusion zone must leave.

"Now we are talking about levels that can impact human health," said Chief Cabinet Secretary Yukio Edano. He told residents: "Please do not go outside. Please stay indoors. Please close windows and make your homes airtight."Don't turn on ventilators. Please hang your laundry indoors."Japan also announced a 30-km no-fly zone around the reactors to prevent planes spreading the radiation further afield. Radiation levels around Fukushima for one hour's exposure rose to eight times the legal limit for exposure in one year, said the plant's operator, the Tokyo Electric Power Co (Tepco).The International Atomic Energy Agency said after Tuesday's blast that radiation dosages of up to 400 millisieverts per hour had been recorded at the site.Exposure to over 100 millisieverts a year is a level which can lead to cancer, according to the World Nuclear Association.On Monday, a hydrogen blast at the Fukushima plant's reactor 3 was felt 40km (25 miles) away. It followed a blast at reactor 1 on Saturday.All explosions have followed cooling system breakdowns. Engineers are trying to prevent meltdowns by flooding the chambers of the nuclear reactors with seawater.Japan's nuclear safety agency said it suspects Tuesday's blast may have damaged reactor 2's suppression chamber.The BBC's Chris Hogg in Tokyo says that would make it a more serious incident than the previous explosions, which were thought just to have damaged the buildings housing the reactors.

The latest official death toll stands at about 2,400 - but some estimates suggest at least 10,000 may have been killed. Thousands are still unaccounted for - including hundreds of tourists - while many remote towns and villages have not been reached.More than 500,000 people have been made homeless.The government has deployed 100,000 troops to lead the aid effort.The UK Foreign Office has updated its travel advice to warn against all non-essential travel to Tokyo and north-eastern Japan. British nationals and friends and relatives of those in Japan can contact the Foreign Office on +44(0) 20 7008 0000.